Origin of Middle Triassic high-K calc-alkaline granitoids and their potassic microgranular enclaves from the western Kunlun orogen, northwest China: A record of the closure of Paleo-Tethys

摘要

The western Kunlun orogen, the northwest Tibet Plateau, occupies a key tectonic position at the junction between the Pan-Asia and the Tethyan tectonic domains. It is related to subduction of Proto- and Paleo-Tethys and terrane collision from early Paleozoic to early Mesozoic This paper presents the first detailed SHRIMP zircon U-Pb chronology, mineral chemistry, major and trace element and Sr-Nd-Hf isotope geochemistry of three Middle Triassic plutons (Yuqikapa, Muztaga and Taer) and their microgranular enclaves in the western Kunlun orogen. SHRIMP zircon U-Pb dating shows that the Yuqikapa pluton was emplaced in the Anisian (～243 Ma) and the Muztaga and Taer plutons that contain microgranular enclaves were emplaced in the Ladinian (234-227 Ma). The Yuqikapa pluton is composed of peraluminous high-K calc-alkaline two-mica granites (monzogranite-syenogranite-alkali-feldspar granite), with initial ~(87)Sr/~(86)Sr ratios of 0.7071-0.7077, ε_(Nd) (T) of -4.4 to -6.3 and ε_(Hf) (T) (in-situ zircon) of—3.3. Elemental and isotopic data suggest that they were formed by "wet" anatexis of the Precambrian metasedimentary and metaigneous rocks in the continental collision (overthrust) zone. The Muztaga and Taer plutons consist of metaluminous to weakly peraluminous high-K calc-alkaline granitoids (granodiorite-monzogranite), with initial ~(87)Sr/~(86)Sr ratios of 0.7052 - 0.7104, ε_(Nd) (T) of - 3.6 to - 7.3 and ε_(Hf) (T) (in-situ zircon) of—3.1 to —1.0. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metasedimentary and metaigneous rocks in the normal lower-crust (<40 km) for the Muztaga pluton and in the thickened lower-crust (～40-50 km) for the Taer pluton, triggered by underplating of coeval (233-226 Ma) mantle-derived enclave-forming magmas. The enclaves are basic (SiO2 48.0-51.6 wt.%) to intermediate (SiO2 53.0-54.6 wt%) with high K2O (1.4-4.2 wt%). They generally have lower initial ~(87)Sr/~(86)Sr ratios (0.7034-0.7084) and high initial ε_(Hf) (—1.1 to +1.2) than their respective host granitoids. The enclave magmas are considered to have been derived from the partial melting of subduction-modified mantle in the spinel stability field (<60 km) (enclaves in the Muztaga pluton) and the spinel-garnet transition zone (-60-80 km) (enclaves in the Taer pluton). Final closure of the Paleo-Tethys occurred in the Middle Triassic Anisian (-243 Ma), The Middle Triassic Ladinian (234-227 Ma) dates the post-collisional regime caused by Paleo-Tethyan oceanic slab break-off.